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Tetralogy of Fallot (ToF)

Tetralogy of Fallot is the most common cyanotic congenital heart disease, with an incidence of approximately 3–5 cases per 10,000 live births, accounting for about 5–10% of congenital heart defects. It is more frequent in males than in females and can occur in isolation or in association with genetic syndromes, such as DiGeorge syndrome (22q11 deletion), trisomy 21 (Down syndrome), and Alagille syndrome.

Tetralogy of Fallot develops due to a defect in the conotruncal division of the bulbus arteriosus during cardiac development, between the 4th and 8th week of gestation.
This defect leads to an anterior and superior displacement of the infundibular septum, resulting in the four characteristic anomalies (tetra):


This combination of defects alters pulmonary and systemic blood flow, causing a right-to-left shunt, which leads to cyanosis and hypoxemia.

Pathophysiology

The key feature of Tetralogy of Fallot's pathophysiology is right ventricular outflow tract obstruction, which increases right ventricular pressure. As a result, blood is diverted through the ventricular septal defect directly into the aorta, bypassing the lungs and reducing blood oxygenation.
The severity of cyanosis depends on the degree of pulmonary stenosis. In mild cases, pulmonary resistance is lower than systemic resistance, and most blood flows into the lungs, leading to minimal cyanosis. This condition is also known as "pink Fallot."
In severe cases, pulmonary resistance is significantly elevated, and most of the blood is shunted into the aorta, causing marked cyanosis.
In newborns with associated pulmonary atresia, pulmonary blood flow occurs only through the ductus arteriosus, making the condition ductus-dependent.
Chronic hypoxemia induces increased erythropoietin production, leading to polycythemia and increased blood viscosity, which may predispose to thrombotic complications.

Clinical Manifestations

The severity of symptoms varies based on the degree of pulmonary stenosis. Cyanosis is the most characteristic clinical sign and may appear at birth or during the first months of life. In newborns with mild stenosis, cyanosis may be minimal or absent, with symptoms developing progressively.
In severe cases, episodes of hypoxic spells may occur, characterized by intense cyanosis, tachypnea, irritability, and possible syncope, often triggered by crying or exertion. These episodes result from a sudden increase in pulmonary stenosis, which worsens the right-to-left shunt. Other symptoms include fatigue, exertional dyspnea, failure to thrive, and digital clubbing. Older children often adopt the characteristic squatting position, which increases venous return and improves pulmonary perfusion.

Diagnosis

Tetralogy of Fallot can be diagnosed prenatally.
Fetal echocardiography, performed between the 18th and 22nd week of gestation, is the primary tool for prenatal diagnosis.
Early detection of the condition allows delivery planning at a specialized center, optimizing neonatal management, especially in critical cases.
The most suggestive ultrasound findings include a large ventricular septal defect, overriding aorta, significant pulmonary stenosis, and, in advanced cases, right ventricular hypertrophy.
However, prenatal diagnosis has some limitations, as a full hemodynamic assessment is challenging due to the characteristics of the fetal circulation.
After birth, the diagnosis is confirmed through Doppler echocardiography, cardiac magnetic resonance imaging (CMR) in complex cases, and cardiac catheterization for patients undergoing surgery.

Treatment

The definitive treatment is surgical correction. In ductus-dependent neonates, prostaglandin E1 may be required to keep the ductus arteriosus patent. Surgical intervention includes: In critically ill neonates, a palliative procedure such as the Blalock-Taussig shunt can be performed to increase pulmonary blood flow and delay full correction.

After surgical repair, long-term survival exceeds 90%. However, residual pulmonary insufficiency, arrhythmias, and right ventricular dysfunction may develop, requiring ongoing follow-up.

Conclusion

Tetralogy of Fallot is a cyanotic congenital heart disease with significant hemodynamic consequences. Prenatal diagnosis is essential for timely intervention. Surgical correction ensures good quality of life and favorable prognosis in most patients.
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